In general, in a hydraulic excavator that adopts a hydraulic system to drive an actuator, such as a boom cylinder, by means of hydraulic fluid discharged from a hydraulic pump that is driven by an engine, the displacement of a boom cylinder or the like is measured by a displacement sensor, an AD converter, and a data acquisition system (DAQ), which are separately mounted on the excavator. Accordingly, the number of components of a measuring device for detecting the displacement of the actuator is increased to cause the increase of the manufacturing cost, and it becomes difficult to precisely control the driving of the actuator to deteriorate workability.